Germany expressed concern over the treatment of Chinese artist Ai Weiwei on Tuesday, saying there was a whiff of "persecution" as Beijing landed him with a huge tax bill.

"I am concerned at how the Chinese authorities are treating Ai Weiwei. China sets itself high standards when it comes to developing its own rule of law. It must now show this in practice," said Markus Loening, Berlin's top human rights official.

"The action against Ai Weiwei again gives the impression that political critics are being wilfully persecuted," added Loening.

The statement called on Beijing to allow Ai, recently named the world's most powerful art figure, freedom of both speech and movement.

Earlier Tuesday, Chinese authorities ordered Ai to pay 15 million yuan ($2.36 million) in alleged back taxes in what the vocal rights activist called an effort to "crush" him.

The tax bill came after Ai was released in June from 81 days in secretive police custody, a detention authorities said was over tax evasion but during which he said he was repeatedly interrogated over his calls for human rights.

"They gave a written notice today.... There was no explanation whatsoever. We questioned where this figure came from -- they couldn't give a clear answer," Ai told AFP.

"The notice said I have 15 days to pay. That's about one million a day... if you don't pay they could put you in jail, maybe up to seven years. I really have no idea."

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